This invention relates to transmitting data from intrinsically safe circuits to non-intrinsically safe circuits. More particularly, this invention relates an opto-coupler used to transmit data between the circuits. Still more particularly, this invention relates to circuitry connected to a phototransistor to prevent excess energy from being applied to the opto-coupler.
Opto-couplers comprising an LED and an associated phototransistor fabricated together as a single device are known and are widely used in applications in which DC and AC isolation must be maintained between a signal source and a signal output representing an amplified or processed input signal. Opto-couplers are used in instances in which the isolation provided by capacitance coupling between an input and an output circuit is not sufficient.
One such instance in which capacitance coupling is not sufficient occurs when input and output circuits are operated in hazardous locations. In such instances, it is well known to use opto-coupler circuitry in which an LED receives an input signal to be amplified or otherwise processed and, in so doing, generates an optical output signal representative of the received input signal. The LED is part of a device package that also includes a phototransistor that has a base that receives the optical output of the LED and, in turn, controls the collector current of the phototransistor. The phototransistor may be advantageously operated as an amplifier whose output is developed across a collector resistor.
An opto-coupler operated in this manner provides increased isolation compared to that provided by the capacitor connected transistor amplifier. The isolation provided by an opto-coupler is needed to connect intrinsically safe circuitry to non-intrinsically safe circuits. Intrinsically safe circuits are circuits that operate under a certain low energy level. By operating under a certain energy level, the circuitry is ensured not to generate a spark or sufficient heat to cause an explosion of material in a hazardous environment even if the circuitry fails in some manner. The power level needed to make circuitry intrinsically safe is determined by regulatory agencies such as the UL in the United States, CENELEC in Europe, CSA in Canada, and TIIS in Japan.
In order to be intrinsically safe, the intrinsically safe circuit must have a galvanic isolation barrier between the intrinsically safe circuit and circuitry that is not intrinsically safe. In order to transmit digital data across the galvanic isolation barrier, a large number of safety components are needed. In an opto-coupler, either the LED or the phototransistor is part of the intrinsically safe circuit.
In a common emitter configuration, the phototransistor saturates and an output voltage from the phototransistor is equal to a saturation voltage of a collector of the phototransistor when the phototransistor is activated by the LED. When the phototransistor is inactive, the output voltage is equal to a power supply voltage. In operation the terminals connected to the phototransistor must not exceed two thirds (⅔) of the maximum current, voltage, or power rating of the opto-coupler. To prevent the terminals connected to the phototransistor from exceeding the maximums, a zener diode and a suitably rated fuse connected to a terminal is typically considered sufficient protection for the terminals. A resistor is also commonly added to prevent against the breaking capacity of the fuse. In this configuration, the diode and the fuse are connected to the collector of the phototransistor and the power supply, thereby increasing the number of components. However, the on all three connections increase circuit size. As the cost of components increases and the board size of circuits decreases, there is a need to configure such circuits in a manner that reduces board cost.
The above and other problems are solved and an advance in the art is made by the protection circuitry of this invention. The circuitry of this invention reduces the number of node connections for a phototransistor to two nodes. A single Zener diode, a fuse, and a resistor provide protection. This reduces the number of components needed, which in turn reduces board cost and board area needed.
In accordance with this invention, the power supply connection to a collector of the phototransistor is reduced to a signal node. Collector-emitter current is conducted to an output resistor to toggle a data line. The power supply connection and collector are reduced to a single node using a resistor circuit connected between the power supply connection and collector. The use of the resistor allows the opto-coupler to remain powered by a dedicated supply current.
In a first aspect of this invention, circuitry that prevents an opto-coupler from exceeding a particular power rating is comprised of the following components. A resistor circuit connects a collector of a phototransistor to a power supply. A fuse connects the power supply to the resistor circuit. A diode connected to the fuse and to the resistor. Finally, a fuse resistor is connected to and between the fuse and the diode.
In another aspect of this invention, a resistor may connect the power supply at a first terminal and to the output resistor between the power supply resistor and the diode.
In another aspect of this invention, an output resistor may be connected between the emitter and ground. The output resistor may be a 604 ohms resistor.
In another aspect of this invention the power supply may provide 14 volts to the circuitry.
In another aspect of this invention the resistor circuit may be a 1.0 kilo ohms resistor.
In another aspect of this invention the fuse may be a 63 milliamp fuse.
In another aspect of this invention, the ground resistor may be a 49.9 kilo Ohms resistor.
In another aspect of this invention the fuse resistor may be a 6.2 Ohms resistor.
In another aspect of this invention, the diode is a 16.0 Volts Zener diode.